Strange one this - I have got a 2017 iMac which won't go online. I have tried connecting it to my home network by Ethernet, Powerline and Wifi and in each case it connects fine to the network itself (i.e. I can use my NAS and other devices), but will not connect to the internet. My 2015 MacBook is sitting right next to it and connects just fine, as does my PC, iPad etc. I have also tried connecting the iMac to a hotspot from my iPhone and it's the same - connects fine, but no internet access. It's not a problem with Safari as iTunes and Speedtest don't connect either. A reboot sorts it out for anything unto 5 minutes and then it's dead again. It must be a software issue, but I just can't work out what. This started on 10.4.4 and is the same on 10.4.5. My MacBook on 10.4.5 is fine.